The invention also relates to the use of this material as a catalyst in conversion processes of hydrocarbons, as molecular sieve and ion exchanger.
Zeolites, of both natural and synthetic origin, are alumino-silicates of a porous crystalline nature consisting of a three-dimensional framework of TO.sub.4 tetrahedrons, wherein T can be silicon or aluminum. The presence of trivalent aluminum in tetrahedric coordination in the crystalline lattice determines an excess negative charge which is balanced by ions of alkaline metals (Na, K, Rb, Li and Cs), alkaline earth metals (Mg and Ca), organic ammonium cations or the H ion.
There is a large number of zeolitic phases of which the crystalline structure is for the most part known. Each of these is characterized by the presence of a particular porous system represented by canals and/or cages with regular and well-defined dimensions within the range of 3-15 .ANG.. On the basis of the average dimensions of the pores, zeolites are generally grouped into "small", "medium" and "large" pores. These characteristics make zeolites useful in a wide range of fields such as ion exchange, separation of mixtures of gases or liquids or as catalysts in various conversion reactions of hydrocarbons.
Zeolites are typically prepared by reacting under hydrothermal conditions a reaction mixture containing a silica source, an aluminum source, an organic templating agent and, optionally, a salt of an alkali or alkaline earth metal. The templating agent, which under suitable conditions, has the function of directing the synthesis of the zeolite towards a well-determined structure, is generally selected from amines and organic quaternary ammonium or phosphonic salts.
The nature of the zeolite obtained depends on the composition of the reaction mixture, the reaction conditions and, above all, the nature of the organic templating agent used.
For example, using tetra ethylammonium hydroxide (Et NOH) as templating agent, beta zeolite is obtained (U.S. Pat. No. 3,308,069), whereas using tetra-propylammonium hydroxide (n-Pr.sub.4 NOH), ZSM-5 zeolite is obtained (U.S. Pat. No. 3,702,886). In some cases zeolites with different structures can be synthesized using the same organic templating agent, but with different synthesis conditions (U.S. Pat. No. 4,016,245, EP-293.032, U.S. Pat. No. 4,052,472).